Contributed by Drew Lyon, Endowed Chair, Small Grains Extension and Research, Weed Science, Washington State University
Figure 1. A chickpea field near Pullman, WA infested with common lambsquarters.
Here is my best guess: timing. As in many other aspects of life, timing is often critical for successful weed control with herbicides. I suspect that growers who had their spring pulse crops planted by the first week of April this year saw much better weed control than those who planted later. Soil applied herbicides, and almost all of the herbicides used in pulse crops for broadleaf weed control are applied preplant or preemergence to soil, require adequate rainfall to both incorporate and activate the product. Rainfall is even more important when tillage is not used for incorporating the herbicide into the soil. The last rainfall of one-quarter inch or more occurred on April 5, 2024, in Pullman. I suspect that growers that had taken advantage of a warm, dry period in late March and early April to plant and apply herbicides had good success with their herbicide programs. Those that applied herbicides after this time likely were disappointed in their weed control.
Rainfall in the Palouse and Inland Pacific Northwest comes largely during late fall and winter (Figure 2). Precipitation declines and becomes less predictable as we enter late spring and summer. Herbicides applied to spring-planted crops near planting time are less likely to receive adequate rainfall to incorporate and activate them compared to herbicides applied prior to planting.
Figure 2. Average monthly precipitation, low, and high temperatures for Pullman, WA. Source: usclimatedata.com.
Some of my earliest work at WSU in pulse crops looked at herbicide timing for common lambsquarters and mayweed chamomile (a.k.a. dog fennel) control in chickpeas. You can see the results of these studies by visiting the annual WSU Weed Control Reports. What I observed over multiple years is that applying herbicides as early as possible in the spring ahead of planting spring pulse crops often provided the best broadleaf weed control, and certainly the most consistent weed control over time. If a significant rainfall event occurred just after applying a post-plant preemergence herbicide, the control equaled or exceeded that of the early application, but this was not a common occurrence.
Sulfentrazone (Spartan 4F and other trade names) applied three to six weeks before planting was particularly effective for the control of common lambsquarters and mayweed chamomile in chickpeas. This treatment can be combined with other soil-applied herbicides or postemergence grass herbicides to pick up other weeds of concern. There is a four-month plant back restriction to wheat after applying Spartan 4F, so earlier application of sulfentrazone also reduces the risk of crop injury to fall-planted winter wheat following the pulse crop.
Pyridate (Tough 5EC) is labeled for postemergence broadleaf weed control in chickpeas and lentils. It is particularly effective on common lambsquarters, but I have heard some complaints about its effectiveness this year. I believe there may be several factors at play. One factor may be the price of Tough herbicide. It is not cheap, so growers may be trying to get by with less than optimal rates. Another consideration with many postemergence contact herbicides like Tough is that many growers want to use less water as a carrier than is optimal. The efficacy of postemergence contact herbicides is almost always improved with increasing carrier volume, which increases spray coverage. To avoid as many trips back to the water tank as possible, many growers use ten gallons or less per acre of carrier. Fifteen to 30 gallons per acre is preferred and will provide better weed control. And the third factor that may have been in play is timing. Yes, timing is critical for postemergence herbicides as well! Postemergence herbicides should be applied to small weeds. When weeds get larger, weed control is generally reduced relative to applications made to smaller weeds. If you combine larger weed size, low water carrier volume, and the use of lower herbicide rates, you will likely experience poor weed control.
Choosing the right herbicides is just part of the battle against weeds. Those herbicides also need to be applied at the right time to ensure success. I think 2024 has provided a good visual example of how important herbicide application timing is for effective weed management.
I welcome your thoughts on what worked well for weed control in spring pulse crops in 2024 and what left you disappointed.